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Evolution of MHC diversity in cetacean species environment: implications for the role of a pathogen

Abstract

Previous studies have indicated that the evolution of diversity in the immune system genes responsible for antigen presentation in terrestrial mammals is driven by selection. However, it is not clear whether mammals in the marine environment would experience the same selective pressures as terrestrial mammals, given their different pathogen environment. The diversity and pattern of radiation of exon-2 (peptide binding region) of the DQBl locus of Class II MHC molecule was investigated in a broad range of cetacean species, including an extended sample of six focal species (three Mysticeti and three Odontoceti species). The role of natural selection and of the evolutionary history of this locus was evaluated based on phylogenetic and genetic distance analyses, in order to assess the following hypotheses: i) the marine pathogen environment presents comparable selection pressures to those of the terrestrial environment, ii) social behaviour and structure will affect pathogenic pressure and iii) populations of cetacean species with a world-wide distribution across different habitats and geographic regions are under differential selective pressure. The phylogenetic comparison of the cetacean species was consistent with the trans-species evolution pattern described for terrestrial mammals. Furthermore, high non-synonymous to synonymous substitution rates suggest that polymorphism at this locus in cetaceans is maintained by natural selection. The higher number of trans- species lineages and non-synonymous substitution rates exhibited by social species suggest that social behaviour and social structure may affect pathogenic pressure. Population differentiation according to DQBl locus reveals a contrasting pattern to that inferred by neutral markers (microsatellite DNA) supporting the hypothesis that habitat and geographic regions may place populations under differential selection pressure. The results of the present investigation suggest that the pattern of evolution of the immune response in cetaceans is similar to that in terrestrial mammal species.